The kanji character 筋 primarily represents "muscle," "tendon," "vein," or "line." It can also refer to a "streak," "quality," or "reasoning." At its core, it conveys the idea of a slender, elongated structure or a pathway of some kind, be it physical or abstract.

・On-yomi (音読み): kin (キン) ・Kun-yomi (訓読み): 筋 (suji)

⚫︎ 筋肉（きんにく） Translation: muscle ⚫︎ 筋トレ（すじトレ） Translation: muscle training/workout (short for "筋力トレーニング" - kinryoku toreeningu) ⚫︎ 話の筋（はなしのすじ） Translation: the storyline / the thread of the story

The kanji 筋 is a pictograph. It depicts the lines or veins in muscles. The top part represents the flesh, and the bottom part represents the sinewy lines. Over time, it developed to also signify lines of reasoning or abstract pathways.